This invention relates generally to methods and apparatus for storing cold liquid, and particularly to a method and apparatus for venting pressurized vapor from the vessel to reduce the rate of expansion of cold liquid and prevent overpressurization of the vessel.
Vessels for storing cold liquids can be used in vehicles or they may be stationary. Typically, such vessels are insulated to reduce the rate of warming of the stored liquid. Nevertheless, the vessel will absorb heat from the environment to its contents. When cold liquids such as liquefied natural gas are stored at -260.degree. F. and then are warmed even slightly, they tend to expand in the liquid phase which increases the volume of the liquid and raises the pressure within the vessel. If the warmed liquid completely fills the vessel volume then the pressure increases rapidly.
To prevent the internal vessel pressure from rising to dangerously high levels, a minimum tank ullage can be provided to accommodate the maximum subsequent increase of liquid volume. The 1990 edition of the National Fire Protection Association, Inc. Standard for the Production, Storage, and Handling of Liquefied Natural Gas (NFPA 59A) includes paragraph 4-1.6 on Filling Volume of liquefied natural gas storage tanks and an accompanying FIG. 4-1, that an operator can use to determine the maximum filling volume to avoid overfilling. For example, using this chart an operator can determine that for a tank with a vapor relief valve setting of 200 psig and tank pressure after filling of 50 psig, the vessel can be filled to a maximum 88.0 percent capacity by volume. The 12.0 percent ullage space accommodates expanding liquid to avoid overpressurization of the vessel. If during normal, regular use liquid is withdrawn from the vessel at a rate greater than the volumetric expansion rate, then the 12.0% volume is unused and imposes an economic and volumetric penalty.
Not all tanks are provided with accurate means for determining tank volume and not all operators will have the necessary skill to apply the principles of FIG. 4-1 for a variety of tanks. Errors can occur in assessing internal pressure at filling and confirming pressure relief valve specifications. Further, intentional or negligent failure to abide by NFPA Standards is a possibility since enforcement can be difficult and costly.
NFPA 59A also includes paragraph 4-8 requiring provision for maintaining internal pressure and vacuum of liquefied natural gas tanks by releasing or admitting gas as needed. Pressure relief valves are intended to prevent overpressurization of a tank, but many are not designed to release liquid. Expansion of the cold liquid in an overfilled vessel can damage a pressure relief valve and result in further complications for maintaining internal tank pressures at safe levels. Thus, a method and apparatus for maintaining the level of cold liquid within a vessel is needed which maximizes the amount of cold liquid initially stored in the vessel yet avoids damage to the pressure relief valve by limiting the level of cold liquid below the pressure relief valve.